邮件标题:日期/时间类型

I'm trying to send an appointment into Lotus Notes by using PHPMailer. I setup all headers which are needed and in the Lotus Notes I can really see the new appointment, but only as a new message. If I want to go into calendar I get message about wrong date/type error.

I went through headers which are generated by Lotus (appoinment created from Lotus Notes) and I can see for example for 'StartDateTime':

Data Type: Time/Date ...
//And the content is:
01.01.2015 04:00:00 CET

If I went through headers which I send from PHP I can see for the same parameter:

Data Type: Text RFC822
//And the content is:
"01.01.2015 04:00:00 CET"
//With the additional quotes!

So I think that Lotus Notes can not read it as a date/time because it is represented as a text. If I'm using PHPMailer, how can I setup that header 'StartDateTime' is date type, not a text. Should I setup some content type?

Edit:

My original header setup was:

$mail->AddCustomHeader("DateStart: 01.01.2015"); // Lotus take it like a string "01.01.2015"
$mail->AddCustomHeader("Icon: 5"); // Lotus take it like string: "5"

But today I found a way when I was analysing the GMail source of mail. The way is that header must be inthis format:

$mail->AddCustomeHeader("X-Lotus-Item: 01.01.2015; name=DateStart");
$mail->AddCustomeHeader("X-Lotus-Item: 05.01.2015; name=DateEnd");
$mail->AddCustomeHeader("X-Lotus-Item: 5; name=Icon; type=300"); // I found that 300 means a number

But still I would like to know why? And why 300 means number and 400 is a date I think. Here you can see some headers. Mail (appointment) was sent from Lotus to GMail:

X-Notes-Item: 1;
 name=$PublicAccess
X-Notes-Item: CN=xxxxxx/OU=usr/O=cag;
 name=$AltPrincipal
X-Notes-Item: 1;
 name=$SMTPKeepNotesItems
X-Notes-Item: 1;
 name=WebDateTimeInit
X-Notes-Item: Meeting;
 name=$IconSwitcher
X-Notes-Item: 1;
 name=Alarms
X-Notes-Item: Thu, 15 Jan 2015 23:45:00 +0100;
 type=400; name=$NoPurge
X-Notes-Item: 0;
 name=Encrypt
X-Notes-Item: ;
 name=HasSafeStamp
X-Notes-Item: CN=fslc001/OU=srvc7/OU=srvc/O=cag;
 type=501; flags=0; name=RouteServers
X-Notes-Item: 14-Jan-2015 08:22:15 CET/14-Jan-2015 08:22:16 CET;
 type=401; flags=0; name=RouteTimes
X-Notes-Item: B2401D9B:142AA393-C1257DCD:00287D34;
 type=4; name=$Orig
X-Notes-Item: 1;
 name=$NoteContainsIcal